As disclosed in Japanese Patent Public Disclosure No. 2004-017508, Japanese Patent Publication No. S50-19132 and Japanese Patent Publication No. S43-27488, it has been proposed to perform a method of injection molding in order to unify a sheet-like material, such as a decorative or ornamental sheet, on a surface of a molded article during a molding process and add new value or optional function to the molded article due to the properties of the sheet-like material, such as color, pattern, quality of material and the like. A conventional method of injection molding of this kind exemplifies the following steps of: installing a decorative sheet in an injection mold, injecting a molten resin to fill the mold cavity with the molten resin; and solidifying the resin in the cavity to form an injection molded body simultaneously with integrating the decorative sheet onto a surface of the injection molded body.
In this method, the sheet-like material such as a decorative sheet is pre-shaped, as required, by an appropriate forming process such as vacuum forming to be adapted to the shape of the mold cavity while installed in the mold or before installed in the mold. When installed in the mold, the sheet-like material such as a decorative sheet covers the whole or part of the mold cavity, and the area of the mold cavity that is covered with the sheet-like material corresponds to the surface of the injection molded body onto which the sheet-like material should be integrated. At this time, the remaining portion of the sheet-like material which protrudes outside the mold cavity is clamped between the parting surfaces or dividing surfaces or abutting surfaces of the first and second mold sections, for example, a cavity mold and a core mold forming opposite sides of the mold cavity, so that the sheet-like material should not be displaced by injection pressure.
To take a pair of a cavity mold and a core mold as an example, the sheet-like material installed on the cavity mold extends continuously from a mold cavity on to a parting surface of the cavity mold, via the peripheral edge of the mold cavity of the cavity mold. Then the core mold is closed against the cavity mold and the sheet-like material is clamped between the parting surfaces of the core mold and the cavity mold while the sheet-like material extends in the mold cavity. The sheet-like material extending in the mold cavity is defined by the peripheral edge of the mold cavity. When molten resign is injected into the mold cavity from an injection gate of the core mold, the molten resin flows along a surface of the sheet-like material to the peripheral edge of the mold cavity. Thereby, the mold cavity is filled with the molten resign and the sheet-like material defined by the peripheral edge of the mold cavity.
The sheet-like material installed in the mold cavity is affected by the temperature of the molten resin injected into the mold cavity and also affected by the injection pressure of the molten resin produced to fill the mold cavity up with the molten resin. The injected molted resin tends to apply a large pressure to the sheet-like material, in particular, in the vicinity of the peripheral edge of the mold cavity. Depending on conditions of injection molding, the sheet-like material may deform around the peripheral edge of the mold cavity. Especially when the sheet-like material extends along an elongated peripheral edge of the mold cavity, there is an increased risk of serious damage to the sheet-like material because of uneven dispersion of the molten resin. In the event that the sheet-like material is damaged, the molten resin leaks out of the mold cavity from the peripheral edge thereof and produce a flash between the parting surfaces of the mold sections. In addition, the molten resin may penetrate through the sheet-like material and damage the outer surface of an injection molded article to be produced. When the sheet-like material is made of an olefin-based thermoplastic elastomer (TPO) with soft texture or the like, the above-mentioned disadvantages may cause serious damage to an injection molded article.